Page 34 - Industrial Plants

Fig. 7 - TR00 prospecting (on land) and fault position

Fig. 8 - Electrical
resistivity tomography
– TR02

s.l.m. It falls within the outcrop areas of the Upper The three ERT surveys parallel to the pipeline (TR01,
Globigerina Limestone Member. The purpose of this TR02 and TR03), are laid on the seabed, for a length of
prospection was to verify the response from the 630 m. The numerical modelling was performed by
electrical point of view, of the substrate surfacing as a adopting the electrical resistivity of the sea water as
whole up to depths of over 50 m. Furthermore, as the determined in the laboratory, with a value of 0.19 Ωm
electrode line passes through a tectonic dislocation at 15°C; while for the depth of the seabed, and of the
(Figure 6), it has been possible to verify the response, electrodes, reference was made to the data of the
from a geoelectric point of view, of the fracturing band existing bathymetric survey. The numerical inversion
associated with it. The inversion was carried out by was always carried out using the Res2DInv64 software,
providing for gradual variations in resistivity (L2 norm - L1 norm inversion method (robust). Despite the
smoothness-constrained least squares method), to relatively low residual error values (6-7%), the inversion
better highlight the lithological variations and the produced artefacts (see Figure 8) in the area closest to
fracturing state (residual 2.5%). the coast, presumably due to the incorrect estimate of
The resulting model shows a discrete lateral continuity the depth of the seabed near the shore line.
of the electrical horizons, characterized by low resistivity
bands (5-20 Ωm), alternated with higher resistivity For this reason, it was decided to “cut” the length of
horizons (20-60 Ωm). The low values can be correlated the longitudinal prospections to 560 m, excluding in
to the presence of a pelitic component (marl and marly the elaboration the section close to the shore line and
limestone), while where it tends to increase, it can be that on land. The results thus obtained are more
attributed to the prevalence of the coarse and reliable and consistent with the local lithological
calcareous component (limestone marly). The effect of context. The inversion was carried out always providing
the fault is visible both in the displacement of the for abrupt changes in resistivity (L1 norm - robust) and
electrical/lithological horizons, and in the definition of a the models obtained have residual between 7% and
slightly less resistive sub vertical band, about eight 8%.
meters wide, which can be correlated with the The three resistivity 2D models, parallel to each other
fracturing band. See Figure 7. and spaced 10 m laterally, show good lateral continuity
of the electric horizons. A first horizon, slightly inclined

IndustrIal Plants - May 2022
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